In the chemical and pharmaceutical industry, it is often necessary to produce extremely precise doses of reagents or products in the form of a powder. This is the case, for example, in laboratories in which molecules are tested or compositions are processed by mixing products. Those products are taken from libraries of samples in which the samples are conserved in the form of small quantities of powder stored in small flasks or reservoirs. The product quantity per sample being limited, it is necessary to use only the smallest possible quantity of product in order not to bring about rapid exhaustion of the library.
Some libraries of samples are provided with an automatic powder dispensing device.
There are automatic dispensing devices which comprise a storage reservoir which is provided with a dosage cap whose opening is adjustable. In accordance with an instruction regarding the quantity of powder to be dispensed and an indication of the quantity of powder already dispensed, a control unit controls the dosage cap in order to increase or decrease the opening thereof. The indication of the quantity of powder is provided by a so-called precision balance, the resolution of such a balance being, for example, 1/10th of a milligram or 1/100th of a milligram.
In the document FR 2846632, the indication of the quantity of powder already dispensed is provided in the same manner by weighing means, the weighing means being arranged under a flask receiving the powder.
However, it is difficult to supply quantities of powder of a few tenths of a milligram in a precise and repetitive manner with such devices. This is because generally, when a balance indicates a first mass value of dispensed powder, the quantity of powder dispensed already exceeds the initial instruction. In fact, the balance detects the start of flow of the powder into the flask at the time the powder touches the bottom of the flask, introducing an excessively large discrepancy for the dispensing of very small quantities of powder, even if the balance has a very high level of precision.